The 21 Vine was a Vine posted on June 22nd, 2013. In the time that the video existed, it amassed over 30 million views and over 600,000 likes on Vine. It became a niche pop culture staple receiving many remixes and parodies.

The video shows a young boy being insulted by his older brother, who says "You stupid!" The young boy responds "No I'm Not!" His brother then asks him "What's 9 + 10?" The boy responds with "Twenty-one!"

The video was originally funny because of the boy's insistence that he was not stupid, followed by his failure to respond to a simple math question. However, in 2019, the video became humorous as a post-ironic reference for people who had enjoyed the popular Vine when it was originally posted.

Following the video, one source claimed in an anonymous post that the young boy had run away from home due to bullying he received after the video was posted. A corroborating note was posted with exaggerated spelling errors and a clue left by the boy that read only the phrase "Twenty-one" These rumors turned out to be false and the note left by the boy was deemed fraudulent.